1. Field of the Invention
The invention relates to displays particularly with respect to liquid crystal monochrome and color displays. Such displays typically are of active matrix configuration.
2. Description of the Prior Art
Backlighted liquid crystal displays (LCD), with, for example, dimmable fluorescent backlight and utilizing twisted-nematic (TN) liquid crystal, have been developed to provide flat panel displays for applications such as aircraft instrumentation, laptop and notebook computers, and the like. Such LCDs typically utilize a back electrode structure in the form of a matrix of transparent metal pixels or dot electrodes and a continuous transparent metal front electrode with the liquid crystal material sandwiched therebetween. The front electrode is often denoted as the common or counter electrode. Each pixel electrode is activated through a switch, usually implemented as a thin film transistor (TFT), which is deposited as a field effect transistor (FET). The drain electrode of each TFT is connected to, or actually forms, the pixel electrode with which it is associated. The gate electrodes of the TFTs in each row of the matrix are commonly connected to a gate bus-line for the row and the source electrodes of the TFTs in each column of the matrix are commonly connected to a source bus-line for the column. An image is created in raster fashion by sequentially scanning the gate bus rows while applying information signals to the source bus columns.
Such an arrangement may provide a monochrome display. Color capability is imparted to the LCD by grouping the pixels into color groups such as triads, quads, and the like, in, for example, diagonal or delta element arrays, and providing color filters at the front surface of the LCD to intercept the light transmitted through the respective pixels. For example, triads with primary color RED, GREEN and BLUE filters are often utilized. By appropriate video control of the gate and source buses various colors are generated.
Backlighted monochrome and color LCD displays have the disadvantage that large light transmission losses occur in the transmission of light from the backlight source through the elements of the display. Light is normally absorbed by non-transmissive areas of the display such as by the black matrix. Typically, only approximately 4% of the light from the backlight source emerges at the front of the display. A large light transmission loss of approximately 90% occurs in the LCD cell structure.
Another disadvantage suffered by such displays is anomalous luminance versus off-axis-angle performance of the display. The LCD off-axis luminance performance of white grayscale luminance versus angle indicates that on-state luminance decreases off axis, whereas off-state luminance increases off axis. It would be desirable for the display to exhibit a flat luminance profile over viewing angle for each grayscale level.
Specifically, with respect to color displays, color LCDs are usually manufactured with a uniform cell gap for all color dots across the display active area. Because of the properties of TN color monogap LCDs, a different level of off-state luminance occurs for each of the color dots. This phenomenon results in undesirably high levels of background luminance. The condition is exacerbated when the display is viewed from varying angles since each color dot changes luminance with viewing angle at different rates, some increasing and some decreasing. This aspect of monogap LCD technology results in high levels of background luminance with viewing angle, producing undesirable secondary effects in viewability of display symbology. Additionally, objectionably different chromaticities of background color for various angles of view result.
Specifically, a RED, GREEN, BLUE (RGB) multicolor display requires an illumination source having strong spectral emissions at 435 nm, 545 nm, and 610 nm. It is impossible to obtain minimum background (off) transmission for all three wavelengths utilizing a display configured with a single cell gap. In such a monogap display, emissions from at least two of the three wavelengths leak through the display background resulting in increased background luminance. This, in turn, results in reduced contrast and a chromatic background.
Prior art techniques to improve the luminance output of LCD displays include using more transmissive polarizers, ITO layers, color filters, and the like; providing ideal rubbing and polarizer orientations; increasing the aperture ratios of the pixels; and utilizing high efficiency backlights. More transmissive elements and high efficiency backlights tend to be expensive, hence increasing the cost of the displays. Increasing the pixel aperture ratios tends to decrease the resolution of the display and hence the clarity thereof. Precise alignments of the rubbing and polarizer orientations require utilization of expensive and hence undesirable manufacturing techniques.
Prior art techniques to improve the luminance versus off-axis-angle performance include backlight designs such as directional diffusers and non-imaging backlight construction. Utilization of these structures tend to undesirably increase the cost of the device.
Other prior art techniques to improve the luminance versus off-axis-angle performance include modifications of the LCD cell such as retardation films, cell gap changes utilizing multi-gap filters and half-toning utilizing half-tone elements. The inclusion of retardation films tends to undesirably increase the cost of the display. The multi-gap construction introduces expensive fabrication techniques to the manufacturing process and in addition may increase image retention and flicker. This aspect of the technology is described in co-pending U.S. patent application Ser. No. 07/850,174; filed Mar. 11, 1992; for Elias S. Haim; entitled "Multi-gap Liquid Crystal Color Display With Reduced Image Retention And Flicker." Said Ser. No. 850,174 is assigned to the assignee of the present invention and is incorporated herein in its entirety. The half-tone construction also tends to undesirably increase the cost of the display because of expensive fabrication techniques and in addition results in gray level non-uniformity across the display. This aspect of the technology is described in U.S. Pat. No. 5,319,480 issued to Richard I. McCartney; entitled "Liquid Crystal Half-tone Display With Gray Level Uniformity." Said U.S. Pat. No. 5,319,480 is assigned to the assignee of the present invention and is incorporated herein in its entirety.